My object is coming out as gray despite its assigned color being white. Other objects don't have this issue. What am I missing?
Update: Looking at your blend, the maximum draw type of your triangle is set to solid, for it to be drawn correctly you need to set it to textured. It can be found under the object tab (orange cube icon).

You need to change the actual material colours to affect the render. These can be found under the surface heading on the materials tab rather than the settings heading, or in the node editor.



The viewport colours you are changing are just that, colours used in the viewport to differentiate between objects, and have no affect on the render.